I. Field of the Invention
This invention relates to the field of crystal formation, in particular the invention involves lanthanum orthogallate and a method which allows lanthanum orthogallate to be produced in the form of large perovskite-type single crystals. Specifically, the invention relates to the discovery that single crystals of lanthanum orthogallate may be grown from a pure melt of lanthanum and gallium oxides while controlling the major crystallographic direction of solidification.
II. Description of the Prior Art
The mineral perovskite (CaTiO.sub.3) serves as a prototype crystal structure for a large class of compounds of general formula ABO.sub.3. In this structure, the A element is a large cation while the B element is a smaller cation. In order to maintain charge neutrality in the compound, the sum of the cation valence states should total six. Thus, various combinations of A and B valence states are possible, e.g. 3--3, 2-4, 1-5 and even mixed fractional compositions. The ideal perovskite crystal structure is cubic where 8(A) ions are located at the cube corners, 6(0)ions at the cube faces and a single B ion at the cube center. In a practical situation, most perovskite structures are distorted from cubic to tetragonal, rhombohederal, or orthorhombic crystal forms. The perovskite structure is likely to be generated where A cations are coordinated with 12 oxygen ions and B cations with 6 oxygen. It was shown first by V. M. Goldschmidt in Skrifter Norske Videnskops-Akad, Oslo, I, Mat.-Naturv. K1., No. 8 (1926) that a cubic perovskite is stable only if a tolerance factor has an approximate range of 0.8&lt;t&lt;0.9. Here t is defined by R.sub.A and R.sub.O =t.sqroot.2 (R.sub.B +R.sub.O) where R.sub.A, R.sub.B, and R.sub.O are ionic radii of the A, B, and O ions respectively. For distorted structures, t may have a larger tolerance. The perovskite structure is very important as a basis for semiconductors, thermoelectrics, ferroelectrics, laser hosts, catalysts, and ferromagnetic materials. A modern discussion of these applications is given in the book "Structure, Properties, and Preparation of Perovskite-Type Compounds" by F. S. Galasso, Pergamon Press, New York, 1969. The perovskite related high Tc superconductor oxides of type La.sub.1.85 Ba.sub.0.15 CuO.sub.4-x and YBa.sub.2 Cu.sub.3 O.sub.7-x, which were discovered in 1987, have again brought this structure into technical prominence. ABO.sub.3 compounds other than CaTiO.sub.3 (perovskite) which possess the perovskite crystal structure are alternatively referred to as "perovskite-type", "perovskite-like" or "perovskite-related". As used herein when referring to compositions other than CaTiO.sub.3, the term "perovskite" describes the crystalline structure of such compositions.
The compound LaGaO.sub.3 was prepared first as a polycrystalline powder, characterized, and reported as a perovskite-like structure by S. Geller, in Acta Cryst. 10, 243 (1957). Geller determined by X-ray diffraction, the structure at room temperature to be an orthorhombic crystal with the following unit cell dimensions: a=5.496 A, b=5.524 A, and c=7.787 A. Small single crystals, ca, 1-5 mm in size, were grown from a PbO--B.sub.2 O.sub.3 flux as reported by M. Marezio, J. P. Remeika and P. D. Dernier in Inorganic Chemistry 7, 1337 (1968). These workers again determined the crystals to be orthorhombic with the following lattice constants: a=5.526 A b=5.473A and c=7.767 A. Several years later a reinvestigation of the La.sub.2 O.sub.3 --Ga.sub.2 O.sub.3 systems by solid state reaction showed a similar result for LaGaO.sub.3. See S. Geller, P. J. Curlander and G. F. Ruse in Mat. Res. Bull. 9, 637 (1974) Geller reported measurement of the orthorhombic unit cell as follows: a=5.519 A, b=5.494 A, and c=7.770 A.
The phase diagram of the La.sub.2 O.sub.3 --Ga.sub.2 O.sub.3 system was studied in a preliminary fashion by S. U. Schneider, R. S. Roth and J. L. Waring, J. Research Natl, Bur. Standards 65A [4] 365 (1961). Schneider et al. found that the perovskite phase existed at the stoichiometric 1:1 composition. However, it was not indicated whether this composition or those adjacent within a few mole percent were either congruently or incongruently melting. Generally mixtures of a high melting component (La.sub.2 O.sub.3) with a partially volatile low melting component such as Ga.sub.2 O.sub.3 are difficult to control compositionally at or near the melting point of the stoichiomitric mix. In accordance with the art it would have been expected that use of a pure melt, and crystal formation therefrom, e.g. by the Czochralski growth technique at ordinary pressure would not succeed.
III. Invention Contrasted From the Prior Art
In accordance with the present invention, it has unexpectedly been found contrary to what is suggested by previous investigators that powdered stoichiometric mixtures of La.sub.2 O.sub.3 and Ga.sub.2 O.sub.3 at the 1:1 Ga.sub.2 O.sub.3 composition melt stably and apparently congruently with little or no evolution of Ga.sub.2 O.sub.3 ; that the resulting stable melts of about 1:1 composition are low melting, circa 1675.degree. C. by uncorrected pyrometer, a temperature near the Ga.sub.2 O.sub.3 melting point; that such pure melts can be formed and retained in iridium crucibles for sufficient time to practice the Czochralski method of growth; that such melts can be seeded by an iridium wire to obtain a single perovskite-type crystal near a preferred [110] orientation; and, that seeded growth via oriented single crystals can produce large boules in [110] orientation or other favorable orientations, viz. [100], [010], or [001].
Additionally the prior art (preparation of single crystals by the flux technique) teaches away from the present invention since crystallization normally proceeds in all directions simultaneously away from an initial nucleus.